Method of making an electrical resistor



DE JONG ET 1.

METHOD OF MAKING N ELECTRICAL RESISTOR Filed May 16, 1968 United StatesPatent 3,534,472 METHOD OF MAKING AN ELECTRICAL RESISTOR Martijn deJong, Matheus Jeanne Gerardus Knapen, and

Theodoor Peter Johannes Botden, Emmasingel, Eindhoven, Netherlands,assignors, by mesne assignments, to US. Philips Corporation, New York,N.Y., a corporation of Delaware Filed May 16, 1968, Ser. No. 729,730Claims priority, applicattison Nzgherlands, May 30, 1967,

Int. Cl. H01c 7/00 US. Cl. 29-620 8 Claims ABSTRACT OF THE DISCLOSURE Amethod of using a laser beam of removing resistance material along ahelical path on the outer layer of a resistor to obtain a predeterminedresistance value.

This invention relates to a method of processing a resistor body, inwhich resistance material is removed along a helical path from aresistance layer applied to a cylindrical carrier consisting ofinsulating material and provided at each end with a connecting cap,until the electrical resistance between the caps acquires apredetermined value; and it also relates to an electrical resistorhaving a resis tance layer which has been processed in this manner.

In the most common method of this kind, the resistance material isremoved along the path by means of a fastrotating grinding disc with asharp edge which is moved in the axial direction along the resistor bodythat is held in a rotary holder. This method has the disadvantage thatthe edges of the path being ground are crumbly rather than sharp,resulting in a limit being set to the minimum pitch that can be used.Furthermore, during the grinding process the resistance material isliable to expand over the basic surface of the groove ground in thecarrier, so that the resistor ultimately obtained may becomevoltagesensitive. Another disadvantage inherent in this grinding methodis that the helical path cannot be made to adjoin directly a terminalcap, with the result that the resistance variation obtainable by thegrinding process is less than in the case where the groove could be madeto adjoin a terminal cap.

These disadvantages are avoided in another known method of theabove-mentioned kind in which the resistance material is removed withthe aid of an electron beam focused onto the layer. However, this isoffset by the fact that this other method requires the use of acomparatively bulky equipment, and that the resistor body to beprocessed, together with the means of producing the electron ray, has tobe placed in vacuo. Furthermore there is the risk that, when using highaccelerating voltages for the electron ray, X-rays are produced and anelectric charge is injected into the carrier, which may cause damage tothe surface of the carrier.

An object of the invention is to provide a method of the above-mentionedkind in which the disadvantages of both methods above described areavoided.

The method according to the invention is characterized in that theresistance material is removed along the path by means of anelectromagnetic beam of rays having an energy of at least several wattsand which is focused onto P CC the resistance layer and that, uponrelative displacement of the resistance layer and this beam, the beam iscontinuously emitted by a device for producing the beam by stimulatedemission. An advantageous form of the method according to the inventionis characterized in that the beam is produced in a device having agas-discharge space filled with a gas consisting of a mixture of carbondioxide, nitrogen and at least one of the gases helium and steam, thewavelength of the radiation in the beam being in practice 10.6 microns,while the discharge space is closed by a window occupying almost theentire cross-section, which window is made of a material permeable toinfra-red radiation and is at the same time a reflecting member.

It should be noted that in the manufacture of metal film resistors witha very small tolerance, it is known first mechanically to cut a helicalpath into a metal layer vapour-deposited on the inner wall of a glasstube, until an ohmic value is obtained which is approximately 0.5% lowerthan the ohmic value desired. Subsequently, metal is evaporated at theend of the helical path by means of a radiation beam which isperiodically interrupted in time and obtained from anintermittently-operating device for producing a radiation by means ofstimulated emission, the amount of evaporated metal being such that theresistance value obtained differs less from the desired value than thepermissible tolerance. This method of manufacturing metal film resistorsretains the disadvantage inherent in mechanically forming the helicalpath, while also the use of a beam of rays emitted intermittentlyrequires succeeding impact areas to overlap if the material is to beremoved over a continuous region. In the method according to theinvention which utilizes a beam emitted continuously, a continuousportion of the resistance material is automatically removed.Consequently the velocity of movement between the focused beam and theresistance layer can be higher than would be possible with anintermittent beam for obtaining a continuous path.

In the above-mentioned advantageous embodiment the gas-discharge spacemay communicate with spaces for the supply and removal of gas. Tominimize the space for the equipment required, it is advantageous to usea gasdischarge space which is closed in a gas-tight manner.

The present invention also relates to a device for carrying out themethod according to the invention described hereinbefore. This device ischaracterized by a rotary holder for receiving a resistor body to beprocessed, at gasdischarge device closed in a gas-tight manner forcontinuously producing a beam of electromagnetic radiation having awavelength of approximately 10.6 microns by stimulated emission, whichdevice contains a gas consisting of a mixture of carbon dioxide,nitrogen and at least one of the gases helium and steam and is providedwith an exit window for the beam; this window is permeable to infia-redradiation and is also a reflecting member. Also provided is means forfocusing the emerging beam onto the resistance layer to be processed ona resistor body in the holder; and further means is provided for arelative displacement of the focus of the beam and the resistor body insuch manner that the focus defines a helical path across the resistancelayer.

In order that the invention may be readily carried into effect, severalembodiments thereof will now be described in detail, by way of example,with reference to the accompanying diagrammatic drawing, in which:

FIG. 1 shows one embodiment of a device for processing a resistor bodyin accordance with the invention;

FIG. 2 shows, in part elevational view and in part longitudinal section,a resistor manufactured by this device according to the invention;

FIG. 3 shows a device similar to that of FIG. 1, but which slightlydiffers therefrom and is viewed in a direction transverse to that ofFIG. 1.

In the form of the method according to the invention which will bedescribed with reference to FIG. 1, resistor bodies 1 each comprising acylindrical and preferably ceramic carrier 2 (FIG. 2), which is coveredwith a resistance layer 3 throughout its cylindrical surface and alsohas metallic terminal caps 4 pressed one onto each end, are suppliedsuccessively through a flexible tubing 5 to hollow, rotatably-arrangedclamping members 13 and 14 of a clamping device indicated as a whole by6. The device is movable over a straight guide formed, for example, by aguide path 7, and the device is formed by a carrier plate or slide 8,which co-acts with the guide path 7 and on which two insulatingsupporting columns 9 are placed, possibly in a mutually displaceablemanner, each of which ends into a metallic hollow sleeve 10.

The clamping members 13 and 14 are rotatably supported in the sleeves 10and placed in such manner that their axes are aligned in the directionof movement of the slide 8 over the guide path 7; the distance betweentheir adjacent ends lies between the distances which exist in theresistor body between the adjacent ends and the remote endsrespectively, of the terminal caps 4 of a resistor body 1 suppliedthrough the tubing 5. The clamping members 13 and 14 can be opened andclosed in the radial direction from without in a manner not shown, forexample, by means of control air supplied through tubings 15. A resistorbody supplied via tubing 5 by means of compressed air, is centrallymoved through the clamping member 13 until its front end, which isguided by a supporting plate or supporting groove 16, finds its Way intothe clamping member 14 and is stopped there. This stopping may becaused, for example, due to the clamping member 14 having an innercollar which, in the halfopen condition of this clamping member when aterminal cap 4 can be slipped into the aperture of the member 14adjacent the clamping member 13, further barricades the way to this capbut no longer forms an obstacle in the fully-open condition of themember 14. A resistor body once being fed into the clamping device, theclamping members 13 and 14 are closed and caused to rotate by a drivingdevice (not shown). It is to be noted that it sufiices if only one ofthe clamping members 13 and 14 is driven. The sense of rotation isindicated in FIG. 1, when viewed from the left, in the clockwisedirection. An object of the method according to the inventionillustrated by the device of FIG. 1.is to remove along helical linematerial from the resistance layer 3 on a resistor body 1 held in theclamping device 6 in a manner which is more advantageous than knownhitherto, so that the electrical resistance of the resistance layerbetween the terminal caps 4 of the body acquires a predetermined value.To this end, the device of FIG. 1 includes a device for continuouslyproducing a high-energy beam 21 of electromagnetic radiation having awavelength of approximately 10.6 microns, the device 20 being indicatedhereinafter as iraser for the sake of simplicity. The iraser 20 isformed by a fully closed discharge tube 22 filled with a gas mixtureconsisting of carbon dioxide, nitrogen and at least one of the gaseshydrogen and helium. Inside the tube are two electrodes 23 and 24, forexample, of platinum, the electrical connections of which are ledthrough the cylindrical part of tube 22, which consists of quartz. Thistube is closed at one end by a concave mirror 25 having a maximumreflection factor and at the other end where the beam 21 emerges, by aplane mirror 26 which transmits the radiation to a certain proportion,for example, 50%. When the electrodes 23 and 24 are connected to anadjustable voltage source 27 which can supply a voltage between 20 kv.and 30 kv., a radiation field is built up in the iraser 20 bycontinuously stimulated emission, resulting in the substantiallyparallel beam 21. The beam 21, which may have a diameter between 5 mm.and 10 mm. and which has an energy of at least several watts, forexample 10 watts, is reflected towards the clamping device 6 by means ofa plane mirror 28 and then concentrated into a narrow beam 30 having aminimum diameter between approximately 150,41. and 2501!. by means of alens 29 made of, for example, intrinsic germanium and having a focallength of, for example, 5 cm. The axis of said beam is directed to theaxis of the clamping members 13 and 14, the lens 29 provided withanti-reflection layers being adjustable by means of an adjustablefitting 33 so that, if the point of intersection of said axes liesbetween the clamping members 13 and 14, the smallest diameter of thefocused beam 30 coincides more or less with that portion of theresistance layer on a resistor body held in the clamping members whichis struck by the beam.

A resistor body 1 received in the clamping device 6 after being suppliedthrough the tubing 5 is now processed by the beam 30 as follows: Theslide 8 is coupled to a driving mechanism 18 shown diagrammatically,which displaces the slide '8 from an initial position in which thefocused beam 30 is directed to that part of the terminal cap of aresistor body 1 held in the clamping device which projects from theclamping member 14, at constant speed along the guide path 7 in thedirection to the left in FIG. 1, so that the resistor body 1 is exploredas it were by the focused beam 30 from the clamping member 14 towardsthe clamping member 13. Since the resistor body 1 follows the rotationof the clamping members -13 and 14 present in the sleeves 10, the beam30 describes a helical line across the resistor body having a pitchwhich is given by the ratio between the rotational speed of the clampingmembers 13 and 14 and the speed of the rectilinear movement of the slide8. The material of the resistance layer 3 which is struck by the beamabsorbs energy from the beam 30, this material thus being locallyevaporated, which process is determined not only by the conversion ofheat supplied to the surface of the resistor body 1, but also by thevelocity of movement of the resistance layer 3 relative to the beam 30.The terminal caps 4 consist of a metal having a thickness of 0.1 mm. ormore, which is thus great relative to the depth of penetration of thebeam of rays. The cap to which the beam 30 is directed at the beginningof the processing thus absorbs so little energy from the beam that it isnot influenced. However, as soon as the beam 30 leaves the cap due tothe relative movement of the resistor body 1 and the beam, the localremoval of the resistance layer begins. This local removal takes placealong a steadily lengthening, helical narrow path 19 which adjoins therelevant cap (see FIG. 2), whereby the electrical resistance between theterminal caps 4 of the resistor body is raised. As the path 19 is beingformed, this resistance is measured with the aid of a measuring bridge31 in a measuring device 32, which is electrically connected to thesleeves 10. The measuring device includes means which are not shown butknown per se, which provides an indication signal when a predeterminedresistance value is reached. This indication signal is used to stop theprocessing of the resistance layer 3 on the resistor body in theclamping device 6 and also to initiate further manipulations for thepurpose of feeding a subsequent resistor body 1 through the tubing 5 tothe clamping device 6.

The processing of the resistance layer 3 by the beam 30 may be stoppedin different ways, for example, by switching off, wholly or in part, theelectrical supply for the iraser 20, which is shown in FIG, 1 by anelectrical connection for a control signal between the measuring device32 and the supply voltage source 27, or intercepting the beam 30 or thebeam 21 by means of a screen which is movable, for example,electromagnetically and which transmits the beam of rays only to alimited extent, if at all. For the same purpose, an electricallycontrollable light valve, such as a Kerr-cell, can be used.

A third possibility is neither to switch off the iraser wholly or inpart, nor to intercept the beam of rays emerging therefrom, but toensure by tilting the mirror 28 or dis placing the lens 29 in the axialdirection, for example upwards in FIG. 1, that the beam 30 insofar itcan still strike the resistor body '1, is defocused locally to such anextent, that the material of the resistance layer 3 is no longerinfluenced because of the low energy density. The indication signal fromthe measuring device 32- is also used in a manner not shown in FIG. 1,to reverse the movement of the slide 8, so that the slide returns to theinitial position shown, and is used further to open the two clampingmembers 13 and 14 so that the resistor body 1 just processed is removedthrough a flexible tubing connected to the member 1 4. Furthermore asubsequent resistor body is supplied through the tubing 5 to theclamping device 6 and held in it to undergo thereafter a similarprocessing as the previous resistor body which has in the meantime beenremoved. The whole process can be automated without fundamentaldifficulty.

As an illustrative example can be mentioned that, if the resistancelayer 3 consists of a carbon layer of approximately 1,000 Angstrom thickwhich has been deposited in known manner by dissociation of ahydrocarbon compound, the beam 30 if having an energy of approximately25 watts permits of forming in this resistance layer a continuoushelical path 19 having a width between 1507/. and 200 at a velocity ofapproximately 0.5 metre per second, that is to say the velocity of therelative displacement of the resistance layer 3 and the focused beam 30may be 0.5 metre per second.

A resistor body 1 which has been processed by the device of FIG. 1 isshown in FIG. 2, in which 19 indicates the helical path along which thematerial has been removed from the resistance layer 3 by the beam. Thewidth of this path, which may be, for example, 200 microns, isadjustable by adjusting the lens 29, that is to say by focussing thebeam 30 more or less sharply on the resistance layer 3. The methoddescribed affords the advantage that the edges of the remaining portionsof the resistance layer which bound the path 19 are smooth and hence notcrumbly, such as occurs in grinding.

Furthermore, the surface of the carrier 2 is clean and smooth at thepath 19, the surface of the ceramic carrier 2 has fused in situ and themolten material may slightly cover the edges just mentioned. All thesefactors con tribute to obtaining good reproducibility of the method andto advantageous electrical properties of the resistor manufactured. Dueto the smoothness of the edges bounding the path 19, this path may begiven a pitch which is lower than is permissible in the known grindingmethod. This implies that, starting from resistor bodies having specificdimensions and a given resistance layer, the greater possibility ofvarying the pitch as well as the direct connection of the path 19 to theterminal cap permits of obtaining an assortment of resistors ofdiiferent ohmic values which is larger than that obtainable with theknown grinding method.

With respect to the known method of forming a similar helical path inthe resistance layer by means of a focused electron ray, the methodaccording to the invention aflords several advantages: There is no needto work in vacuo, considerably lower operating voltages are permissible,resulting in a less bulky equipment, greater energy densities can beused without a risk of X-rays ocurring, so that the rate of processingcan be increased, which is advantageous for bulk production.

FIG. 3 shows diagrammatically a portion of a device which differs fromthat of FIG. 1 only in a few details, the same reference numerals havingbeen used for corresponding elements. The rotary clamping members 13 and14 are now fitted on insulating supporting arms 40 and 41 which canrotate about a spindle '44 supported by the slide 8, through an anglebounded by stop cams 42 and 43 and in a plane transverse to the axis ofthe resistor body 1. In the one position of the supporting arms theresistor body 1 finds itself wholly outside the beam 30, in the otherposition the said beam is exactly focused on the resistance layer 3. Theprocessing of this layer by means of the beam 30 can be stopped in thisdevice in a similar manner as in the device of FIG. 1. A furtherpossibility is the use of an electromagnetic operating device 45 coupledto the supporting arms, which device is controlled by the indicatingsignal from the measuring device 32 and then pushes the resistor body 1as it were out of the beam 30. In the device of FIG. 3, the iraser isarranged transversely instead of parallel to the direction of movementof the slide 8. The mirror indicated by 28 in FIG. 1 may then bedispensed with.

It should be noted that, if the relative displacement of the beam 30 andthe resistor body 1 in the axial direction of the latter as is necessaryin the method described is not unduly large, this displacement may beeffected in a manner other than that above described by shifting thelens 29 in the said direction, whereby the slide 8 remains in place andneed not be designed as a slide.

The material referred to above for the resistance layer '3 is carbon.However, the method acording to the invention is not limited thereto.Thus the resistance layer 3 may consist of metal, such as nickel ornickel chromium, insofar the metal layer is not unduly thick. On the onehand there must be suificiently absorption of energy, which is obtainedin thin layers due to the surface of the carrier 2 absorbing theradiation of the beam 30 transmitted by the metal layer, on the otherhand the local thermal capacity and dissipation of heat must not be suchas to prevent the temperature rise necessary for the evaporation of themetal. To form a metal-free path of approximately 200 wide, anickel-chromium layer of approximately 1000 Angstrom thick canexcellently be processed in the described manner by the beam 30 havingan energy of 30 watts at a velocity of movement of the beam across thenickel-chromium layer of -approxi-' mately 0.5 metre per second.

What is claimed is:

1. In a method of processing a resistor body initially formed of aresistance layer upon a cylindrical carrier consisting of insulatingmaterial with a connecting cap on each end, the improvement incombination therewith comprising: (a) directing along a helical path onsaid resistance layer an electro-magnetic beam of rays continuouslyemitted by a device for producing the beam by simulated emission, (b)focusing the beam onto the re sistance layer, (c) relatively displacingthe resistance layer and the beam to traverse said path, and (d) therebyremoving resistance material along said path until the electricalresistance between the caps acquires a predetermined value.

2. A method as defined in claim 1 wherein the beam is produced in adevice having a gas-discharge space filled with a gas consisting of amixture of carbon-dioxide, nitrogen and at least one of the gases heliumand steam, the wavelength of the radiation in the beam beingapproximately 10.6 microns, while the discharge space is closed by awindow occupying almost the entire cross-section, which window is madeof a material permeable to infrared radiation and is at the same time areflecting member.

3. A method as defined in claim 2 wherein the gasdischarge space isclosed in the gas-tight manner.

4. A method as defined in claim 1 wherein the resistance layer consistsof carbon deposited on the insulating carrier.

5. A method as defined in claim 1 wherein the resistance layer is formedby a metal layer, for example, a nickel-chromium alloy, having athickness between approximately 20 Angstrom and 3000 Angstrom.

6. A method as defined in claim 1 comprising the further step of varyingthe energy of the beam uponthe resistor body at the beginning and end ofthe method.

7. A method as defined in claim 6 comprising the further steps at thebeginning and termination of the method of defocusing the beam such thata transition occurs on the resistance layer.

8. A method as defined in claim 1, further comprising, directing thebeam to one of the caps prior to removing the material from theresistance layer.

8 References Cited UNITED STATES PATENTS 6/1968 Lins 29-610 JOHN F.CAMPBELL, Primary Examiner

